1. Field of the Invention
This invention relates to a circuit interrupters which provide waveform capture for harmonic analysis of the waveforms in a protected electrical system, as well as overcurrent protection.
2. Background Information
State of the art circuit interrupters, which includes, for instance, circuit breakers, vacuum interrupters, contactors, motor controllers and the like, have a trip unit incorporating a microprocessor. The microprocessor executes algorithms which implement a protection curve. Typically in circuit breakers and vacuum interrupters, the protection curve provides an instantaneous trip, a long delay trip, and if appropriate, a short delay trip function. The microprocessor supervises analog to digital conversion of the analog waveforms as well as executing the protection algorithms. In some circuit interrupters, the microprocessor also performs calculations for metering. All of these functions place a burden on the microprocessor. Thus, the sampling rate must be balanced against the processing time required for the other functions performed by the microprocessor.
While the instantaneous and short delay protection functions respond to the essentially sinusoidal fault currents which occur in an electrical distribution system, the long delay and metering functions are more sensitive to distortions in the waveforms of the electrical system which are becoming more common. Distorted waveforms require more samples per cycle to accurately represent the waveform; however, higher sampling rates reduce the time available for the microprocessor to perform its other assigned functions. One approach to solving this dilemma has been the development of equivalent sampling techniques. In equivalent sampling, the timing between several repetitions of a pattern of samples taken over several cycles is "bumped", so that when all of the samples are superimposed on an equivalent single cycle by waiting until the several repetitions of the pattern have been completed, a higher effective sampling rate is achieved. Our cross-referenced application discloses an equivalent sampling technique in which the timing between successive pairs of samples taken 90 electrical degrees apart is adjusted to produce an equivalent sampling rate of 64 samples per cycle. Each pair of samples provides a measurement of the rms value of the fundamental frequency of the waveform.
There is also a growing interest in the capability of determining the amount of distortion in the waveforms. Monitors have been developed to analyze, and in particular to determine the harmonic content of waveforms in an electrical distribution system. However, according to the Nyquist criteria, a waveform must be sampled at greater than twice the frequency of the highest harmonic to be extracted. This requires very high sampling rates in order to capture the harmonics that are typically of interest. For example, if up to the 50th harmonic is to be extracted, sampling must be performed at the rate in excess of 6 KHz. Furthermore, the Fourier analysis typically used to extract the harmonics requires considerable processing time. The result is that dedicated units have been developed for waveform analysis. These units may do some metering in addition to the waveform analysis, but they do not perform protection functions. The high sampling rate required for the waveform analysis is compatible with providing protection; however, it requires so much computer time that none is left to provide protection. Some of the monitoring units capture the waveform digitally for later analysis, either in the unit or in a remote processor connected to the unit by a communications link. Commonly owned U.S. patent application Ser. No. 08/325,711 filed on Oct. 17, 1994. in the name of Robert Elms discloses an analyzer which uses an equivalent sampling rate for metering and a synchronous rate for waveform capture. This analyzer also calculates harmonic distortion, but it does not provide protection.
There is a need, therefore, for an improved circuit interrupter which can provide waveform capture as well as protection functions.
There is also a need for such an improved circuit interrupter which can perform the harmonic analysis on the captured waveform internally, and which can preferably display the results of the harmonic analysis.
There is an additional need for such an improved circuit interrupter which can perform all of these functions at a competitive cost.